A walkthrough, end to end.
- 1
Enter the first day of your last menstrual period (LMP). This is the most reliable input for an LMP-based estimate.
- 2
Adjust your typical cycle length if it's not 28 days. The calculator shifts the estimated conception date accordingly.
- 3
Read your estimated due date plus the start dates of each trimester and your current gestational age in weeks + days.
Naegele's rule
Add 280 days (40 weeks) to the first day of your last menstrual period. Naegele's rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the conception date shifts but the LMP-based due date is the standard clinical reference.
What you can do with this.
Calculating due date from LMP
If you remember the first day of your last period, the LMP method is the most common starting point. It's what your provider will use at your first prenatal visit before an ultrasound dating scan.
Adjusting for irregular cycles
Naegele's rule assumes a 28-day cycle. If yours is consistently 30 or 35 days, your true conception date is later than day 14 — your provider may revise the due date with an early ultrasound.
First, second, third trimester dates
Trimesters are standardized: first runs LMP–13w6d, second 14w0d–27w6d, third 28w0d–delivery. Knowing these helps you plan around screening windows like the NIPT (10–12w) and anatomy scan (~20w).
Due date calculator 2026 — what's standard now
ACOG (2026) still recommends LMP-based dating in the first trimester only when an early ultrasound (≤13w6d) confirms it within 5–7 days. After that window, ultrasound takes precedence. Use this calculator as a personal reference, not a replacement for your provider's official dating.
Frequently asked.
About 5% of babies arrive on their exact due date. Most births happen within 2 weeks before or after. An early-trimester dating ultrasound is more accurate than LMP if your cycle is irregular.
Use a conception date or an ultrasound dating scan instead. Your provider can date the pregnancy from a first-trimester ultrasound to within 5–7 days.
Naegele's rule (1830s) added 7 days and subtracted 3 months from LMP — equivalent to 280 days. Modern data shows the average is closer to 282 days for first-time mothers, but 280 remains the convention.
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